P0670 fault

[Gt1984]

Evening, recently purchased A6 2018 2.0TDI and had the glow plug relay changed twice after the EML came on with the p0670 fault code. Clearly it's not two new relays, the fault once cleared comes back after about 3-4 engine starts which suggests it is an intermittent fault. The last autoelec that checked said all 4 Glow plugs are fine and also the GP fuse is GOOD changed the relay and light came back on😭. I'm thinking to get the GP fuse changed anyway not sure of location of the fuse if anyone knows the location? or any other suggestions would be greatly appreciated. 

Audi A6 SE 2.0 TDI 90ps 

38000mls